1. Field of the Invention
This invention relates to overbased calcium sulfonate greases made with added calcium hydroxyapatite as a base source and the method for manufacturing such greases to provide improvements in both thickener yield and expected high temperature utility as demonstrated by dropping point, even when the oil-soluble overbased calcium sulfonate used to make the grease is considered to be of poor quality.
2. Description of Related Art
Overbased calcium sulfonate greases have been an established grease category for many years. One known process for making such greases is a two-step process involving the steps of “promotion” and “conversion.” Typically the first step (“promotion”) is to react a stoichiometric excess amount of calcium oxide (CaO) or calcium hydroxide (Ca(OH)2) as the base source with an alkyl benzene sulfonic acid, carbon dioxide (CO2), and with other components to produce an oil soluble overbased calcium sulfonate with amorphous calcium carbonate dispersed therein. These overbased oil-soluble calcium sulfonates are typically clear and bright and have Newtonian rheology. In some cases, they may be slightly turbid, but such variations do not prevent their use in preparing overbased calcium sulfonate greases. For the purposes of this disclosure, the terms “overbased oil-soluble calcium sulfonate” and “oil-soluble overbased calcium sulfonate” and “overbased calcium sulfonate” refer to any overbased calcium sulfonate suitable for making calcium sulfonate greases. Typically the second step (“conversion”) is to add a converting agent or agents, such as propylene glycol, iso-propyl alcohol, formic acid or acetic acid, to the product of the promotion step, along with a suitable base oil (such as mineral oil), to convert the amorphous calcium carbonate to a very finely divided dispersion of crystalline calcium carbonate. Because an excess of calcium hydroxide or calcium oxide is used to achieve overbasing, a small amount of residual calcium oxide or calcium hydroxide may also be present and will be dispersed. The crystalline form of the calcium carbonate is preferably calcite. This extremely finely divided calcium carbonate, also known as a colloidal dispersion, interacts with the calcium sulfonate to form a grease-like consistency. Such overbased calcium sulfonate greases produced through the two-step process have come to be known as “simple calcium sulfonate greases” and are disclosed, for example, in U.S. Pat. Nos. 3,242,079; 3,372,115; 3,376,222, 3,377,283; and 3,492,231.
It is also known in the prior art to combine these two steps, by carefully controlling the reaction, into a single step. In this one-step process, the simple calcium sulfonate grease is prepared by reaction of an appropriate sulfonic acid with either calcium hydroxide or calcium oxide in the presence of carbon dioxide and a system of reagents that simultaneously act as both promoter (creating the amorphous calcium carbonate overbasing by reaction of carbon dioxide with an excess amount of calcium oxide or calcium hydroxide) and converting agents (converting the amorphous calcium carbonate to very finely divided crystalline calcium carbonate). Thus, the grease-like consistency is formed in a single step wherein the overbased, oil-soluble calcium sulfonate (the product of the first step in the two-step process) is never actually formed and isolated as a separate product. This one-step process is disclosed, for example, in U.S. Pat. Nos. 3,661,622; 3,671,012; 3,746,643; and 3,816,310.
In addition to simple calcium sulfonate greases, calcium sulfonate complex grease compounds are also known in the prior art. These complex greases are typically produced by adding a strong calcium-containing base, such as calcium hydroxide or calcium oxide, to the simple calcium sulfonate grease produced by either the two-step or one-step process and reacting with stoichiometrically equivalent amounts of complexing acids, such as 12 hydroxystearic acid, boric acid, acetic acid, or phosphoric acid. The claimed advantages of the calcium sulfonate complex grease over the simple grease include reduced tackiness, improved pumpability, and improved high temperature utility. Calcium sulfonate complex greases are disclosed, for example, in U.S. Pat. Nos. 4,560,489; 5,126,062; 5,308,514; and 5,338,467.
All of the known prior art teaches the use of calcium oxide or calcium hydroxide as the sources of basic calcium for production of calcium sulfonate greases or as a required component for reacting with complexing acids to form calcium sulfonate complex greases. The known prior art generally teaches that the addition of calcium hydroxide or calcium oxide needs to be in an amount sufficient (when added to the amount of calcium hydroxide or calcium oxide present in the overbased oil-soluble calcium sulfonate) to provide a total level of calcium hydroxide or calcium oxide sufficient to fully react with the complexing acids. There are also prior art references for using tricalcium phosphate as an additive in lubricating greases. For instance, U.S. Pat. Nos. 4,787,992; 4,830,767; 4,902,435; 4,904,399; 4,929,371 all teach using tricalcium phosphate as an additive for lubricating greases. However, it is believed that no prior art references teach the use of calcium hydroxyapatite, Ca5(PO4)3OH, as a calcium-containing base for reaction with acids to make lubricating greases, including calcium sulfonate-based greases. The known prior art also generally teaches against the use of calcium carbonate (as a separate ingredient or as an “impurity” in the calcium hydroxide or calcium oxide, other than the presence of the amorphous calcium carbonate dispersed in the calcium sulfonate after carbonation) in making calcium sulfonate greases for at least two reasons. The first being that calcium carbonate is generally considered to be a weak base, unsuitable for reacting with complexing acids. The second being that the presence of unreacted solid calcium compounds (including calcium carbonate, calcium hydroxide, calcium oxide, or calcium hydroxyapatite) interferes with the conversion process, resulting in inferior grease compounds if the unreacted solids are not removed prior to conversion or before conversion is completed.
Additionally, the prior art does not provide a calcium sulfonate complex grease with both improved thickener yield and dropping point. The known prior art requires an amount of overbased calcium sulfonate of least 36% (by weight of the final grease product) suitable grease in the NGLI No. 2 category with a demonstrated dropping point of at least 575 F. The overbased oil-soluble calcium sulfonate is one of the most expensive ingredients in making calcium sulfonate grease, therefore it is desirable to reduce the amount of this ingredient while still maintaining a desirable level of firmness in the final grease (thereby improving thickener yield). Specifically, it is desirable to have an overbased calcium sulfonate grease wherein the percentage of overbased oil-soluble calcium sulfonate is less than 36% and the dropping point is consistently 575 F or higher when the consistency is within an NLGI No. 2 grade (or the worked 60 stroke penetration of the grease is between 265 and 295). Higher dropping points are considered desirable since the dropping point is the first and most easily determined guide as to the high temperature utility limitations of a lubricating grease.